Use of lower voltages and higher currents in switching power supplies is now a definite trend that is attendant upon a decline in voltage and increase in capacity of the internal power supplies of information devices in recent years.
FIG. 1 is a perspective view useful in describing the typical structure of an inductor used in a switching power supply, and FIG. 2 is a bottom view of the inductor of FIG. 1. By way of example, an example of the structure of such an inductor is described in the specification of Japanese Patent Application Laid-Open No. 11-307364.
An inductor 1 of the kind shown in FIG. 1 includes a columnar core 3 (this will be referred to simply as a “core 3” below) of a bobbin 2 comprising an electrical insulator. After a coil 4 is wound upon the core 3, both ends of the coil are connected to pin-shaped terminals. Furthermore, the core 3 is a hollow columnar body. As shown in the bottom view of the inductor 1 of FIG. 2, well-known E-shaped cores 6 (each referred to simply as a “core 6” below) are inserted symmetrically into two insertion openings 5 of the core 3.
Further, FIG. 3 is a schematic view illustrating a development of the surface of the core 3. The coil 4, which is formed by wire having a certain wire width, indicates an area occupied on the core surface. It should be noted that the wire width is equal to the width dimension of the wire projected onto the core surface when the wire is wound upon the core. For example, if the cross section of the wire is rectangular and the wire is wound in such a manner that one side thereof contacts the core, as illustrated in the schematic view of the core cross section shown in FIG. 4, the wire width will be equal to the width dimension of the wire. Similarly, if the cross section is circular, then the diameter will be the wire width. The wire width, therefore, is δ2 in FIG. 3. The cross-sectional shape of the core 3 is represented by a square shape for explanatory purposes.
In a conventional low-voltage, large-current inductor, wire having a large cross section is used for the coil in order to lower resistance to conduction. The conventional inductor 1 shown in FIG. 3 is such that when the wire is wound upon the core, wire having a wire width that will just fall within the length (winding-area width W) of the core along the axial direction thereof is selected appropriately to thereby assure that the cross-sectional area of the wire will be as large as possible.
It should be noted that A and B in FIG. 3 respectively illustrate a position at which the wire first makes contact with the core 3 and starts being wound upon the core 3, and a position at which winding of the wire ends and departs from the core 3. These positions will be referred to as winding starting (and end) positions. The length of one wire that occupies the core along the axial direction thereof when the wire has been wound is defined as the wire pitch (also referred to simply as “pitch”). The pitch of the wire of coil 4 in FIG. 3, therefore, is δ2. Furthermore, the direction along which the wire is wound upon the core is defined as the winding direction of the core. In FIG. 3, this indicates a direction orthogonal to the axial direction of the core.
However, even if wire having a large cross section is used, as described above, an empty space remains in the vicinity of the winding starting position or wiring end position on the core 3. FIG. 5 is an enlarged schematic view of the core 3. Here the area other than the area occupied by the coil 4 in FIG. 3 is represented by a hatched area X2. As indicated by the hatched area X2 in FIG. 5, an empty space remains near the winding starting position or wiring end position on the core 3 and the area factor (area occupation ratio) of the winding area (W) by the coil declines. The result is an inductor of greater size.